Having trouble connecting to O2 wifi hotspots? Unistall the O2 wifi app, using another provider's sim re-install the app and enter that mobile number when registering. After receiving you new code and it being accepted, swap back to giffgaff and all should be well :-)
Other than locating hotspots, I'm not sure what benefit the App actually brings. O2 WiFi is actually set up so that you don't need usernames or passwords, and once you're registered, you should always be able to automatically connect.
Also, as we're talking about WiFi, I'm not sure how using another provider's sim card could actually help in this situation. If I'm understanding you correctly, are you suggesting registering a different mobile number to register for O2 WiFi, before switching back to your original giffgaff sim?
If that works, then all well and good, but I'm not convinced that you need to go to all that trouble.